Motors have conventionally been incorporated in various types of apparatus. Motors rotate a driven target by causing an electric current to flow through a motor coil and thus generating a rotation force (i.e., torque) corresponding to the size of the electric current. When a motor is rotated at a constant speed, it is sufficient to generate a torque corresponding to a load on a bearing or the like. Accordingly, only a small amount of current is required. However, when the speed of rotation needs to be rapidly accelerated or decelerated, a torque corresponding to the magnitude of the acceleration of deceleration is required. In such a case, an electric current of an amount corresponding to the required torque needs to flow. The heat generated in the motor coil by the flow of an electric current increases in proportion to the square of the size of the electric current. Therefore, the motor coil in an apparatus incorporating a motor which is rotated with rapid acceleration or deceleration generates a huge amount of heat, and accordingly it is required to prevent a temperature rise caused by the heat generation.
For example, a disk apparatus, for performing information recording and reproduction by rotating a disk as a recording medium while moving a recording head, sometimes repeats a seek operation frequently. The seek operation means the operation of moving the recording head to a desired position over the recording medium at high speed. During the seek operation, the rotation speeds of a head transporting motor for transporting the recording head and a disk motor for rotating the disk need to be rapidly changed. These motors generate a huge quantity of heat. Such heat generation caused during the seek operation raises the temperature of the disk and the elements of the disk apparatus to a level exceeding the temperature at which the disk and the elements can withstand. Solutions of this problem of temperature rise have been proposed.
Japanese Laid-Open Publication No. 6-119008, for example, discloses a system including a temperature sensor in an optical disk apparatus, which operates as follows. The temperature of the optical disk apparatus is detected. When the temperature of the optical disk apparatus exceeds a preset temperature, the operation of the optical disk apparatus is restricted, so that an excessive temperature rise is prevented.
In order to reduce the cost of components and the number of steps of assembly which are required by the provision of a temperature sensor, or in order to detect the temperature at a position at which it is difficult to directly attach a temperature sensor, technology for predicting the temperature at a desired position by calculation has been proposed. Japanese Laid-Open Publication No. 7-153208, for example, discloses a system for predicting, by calculation, a temperature rise of a voice coil motor (VCM), as a head transporting motor, based on a value of the electric current commanded to the VCM.
FIG. 10 shows a conventional magnetic disk apparatus 101 for predicting a temperature rise by calculation. The magnetic disk apparatus 101 includes a servo controller 118 and a disk enclosure (hereinafter, referred to as the “DE”) 102. The servo controller 118 includes a VCM control section 135. The VCM control section 135 includes a RAM 122, a positioning control section 115, and a temperature detection section 114 for predicting the temperature of a VCM 106 (described below). The DE 102 includes a disk motor 103, a spindle 104, a magnetic disk 105, the VCM 106, and a magnetic head 107. The magnetic head 107 is moved in a radial direction of the magnetic disk 105 by the VCM 106. Thus, the magnetic head 107 is properly positioned.
In the RAM 122, various data is stored. The data to be stored in the RAM 122 includes, for example, iv (value of the electric current commanded to the VCM 106), ΔQv1 (heat quantity corresponding to the temperature rise), ΔQv2 (heat quantity spontaneously radiated), Qv (heat quantity of a measurement target), and Tv (temperature of the measurement target). The measurement target is an element, the temperature of which is to be measured. The data stored in the RAM 122 can be updated. A timer (soft timer) is set in the RAM 122. A ROM (not shown) included in the VCM control section 135 has various data already stored therein. The data stored in the ROM includes, for example, K (constant), θ (constant of thermal resistance), Cv (thermal capacity of the measurement target), Te (environmental temperature), ts (sampling time), a (constant) and b (constant).
In the disk apparatus 101 having the above-described structure, the temperature detection section 114 predicts the temperature of the VCM 106 by calculation in the following manner.
The VCM control section 135 interrupts a usual seek control every 66 μsec. (i.e., sampling time ts), and detects the position of the magnetic head 107 and updates the value iv.
Next, the temperature detection section 114 multiplies the square of the value iv by coefficient K and sampling time ts. Then, ΔQv2 is subtracted from ΔQv1. The resultant value is integrated (i.e., Qve←Qv+ΔQv1−ΔQv2) to obtain the heat quantity Qv of the measurement target. Thus, the temperature Tv of the measurement target is detected (Tv=Qv/Cv).
The above-described processing is performed every 66 μsec. (i.e., sampling time ts) to detect the temperature Ts. The detected temperature Ts is stored in the RAM 122. When the seek operation is performed, the temperature Tv is read from the RAM 122, and the seek operation is controlled based on the temperature Tv.
When the detected temperature Tv is higher than a reference value, the start of the seek operation is delayed in accordance with the temperature Tv. Thus, the temperature rise is restricted.
A delay amount D′, by which the start of the seek operation is delayed, is expressed by D′=a·Tv−b as the linear function of the temperature Tv (where a and b are constants stored in the ROM).
The delay amount D′ is set as follows in accordance with the temperature Tv. The reference value for the temperature is T1′. In a region where Tv≦T1′, D′ is set as 0; and in a region where Tv>T1′, D′ is set as a·Tv−b.
Accordingly, when the temperature Tv is equal to or lower than the reference value T1′, the seek operation is started immediately upon receiving an instruction to seek. When the temperature Tv is higher than the reference value T1′, the start of the seek operation is delayed by the delay amount which is set in proportion to the temperature Tv. In this manner, the temperature rise of the VCM 106 is restricted, and thus the VCM 106 is protected against overheating.
However, the conventional magnetic disk apparatus 101 has a problem in that the precision of calculation of the heat quantity ΔQv1 is significantly poor.
The heat quantity ΔQv1 can be found by multiplying the square of the amount of the electric current flowing through the coil of the VCM 106 by a constant (coil resistance and the time during which the electric current flows). In the above-described conventional apparatus 101, the calculation is performed using the value of the electric current commanded to the VCM 106 instead of the amount of the electric current itself. This is done with the premise that the amount of the electric current and the value of the electric current commanded to the VCM 106 are proportional with respect to each other. However, these two are not necessarily proportional to each other.
FIG. 11 is a graph illustrating the relationship between the value iv and the actual amount of the electric current i of a general motor driver IC. As shown by the chain line in FIG. 11, the conventional calculation method assumes that the value iv and the actual amount of the electric current i are proportional to each other (i=c·iv). However, there is actually a range of values iv, which is referred to as the “dead zone”. In the dead zone, the amount of the electric current i which is output is zero regardless of the value iv. The size of the dead zone greatly varies among individual ICs, so that the value iv and the actual amount of the electric current i are not proportional to each other. Furthermore, in a range where the value iv is larger than a constant iv0, the relationship between values iv and i (i.e., i=c1·iv+d1) becomes i=c2·iv+d2. Therefore, the relationship between values iv and i exhibits non-linearity in that the proportionality factor (c1 and c2) varies in accordance with the range of the value iv. Additionally, constants c1, c2, d1 and d2 greatly vary among individual ICs. The value i can vary significantly with respect to the same value iv, depending on the IC.
The driver IC detects the value i and the feedback control by monitoring voltages at both ends of a detection resistor provided in series to the motor. However, the resistance of the detection resistor is set to be as small as about 0.1 Ω in order to minimize the motor driving loss. Therefore, the influence of errors of the line resistances in the IC or the like is not negligible. As a result, it is difficult to raise the precision of current detection.
For the above-described reasons, the electric current icalc calculated by the conventional method based on a value iv1 has a large error with respect to an actual electric current ireal. The heat quantity ΔQv1 calculated from the amount of the electric current icalc differs substantially from the heat actually generated in the VCM 106. Thus, the precision of temperature prediction is very poor.
Since the VCM 106 is controlled by the result of temperature prediction having such a poor precision, the following inconveniences occur: the start of the seek operation is not delayed although the actual temperature exceeds the reference value T1′, which results in overheating or destruction of the VCM 106; and the start of the seek operation is delayed although the actual temperature is sufficiently lower than the reference value T1′, which lowers the performance of the magnetic disk apparatus 101.
A direct detection of the actual electric current i at a high precision requires a complicated structure and undesirably raises the cost of components and the number of steps of assembly.